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Grid Altitudes on charts

Just wondering whether it is stated anywhere on charts what exactly the altitude bound in the lat/long grids represents?

Reason for asking is that I have 2 charts for Portugal. The NAA Issued one, and the Jepp one. I am led to believe that the Jepp one gives you a grid MORA, i.e. including the obstacle clearance. Whereas on the NAA one it gives different figures... I have been given 2 opinions on this; that it is highest obstacle +500', and also that it is merely the height of the highest obstacle.

Despite my searching I can't find on the chart legend anything that confirms this either way, and I would really like to know for sure what exactly the chart is telling me.

Whatever the exact number, it will be based on the highest of the following two numbers:

  • The highest terrain elevation plus 300' or 500' (to account for an unknown obstacle on that point).

  • The top of the highest known obstacle

Depending on the chart maker they then add the safety margin (typically 1000') and round it up to the nearest 100'.

On the back of the Jeppesen chart I managed to pull from the pile in my office there's a full description on the backside of the chart.

I think this varies according to chart.

Look here for example (page 20 of the PDF) for a fine example of non-consistency between different charts.

Also I think some charts (the UK CAA ones, possibly) include a margin of some 1 or 2 miles outside each sector, to account for obstacles which lie right on the line.

The legend (the descriptive text) on each chart should give the details, but mostly they don't give the exact rules for obstacle inclusion.

Administrator
Shoreham EGKA, United Kingdom

I suspect (but don't know for sure) that Jepp are using the FAA system, which is something like highest obstacle rounded up to nearest 500', plus 200' margin added. There's also a provision for man-made obstacles sited on the highest point of terrain. Don't have time to look up the details right now.

I assume that the above refers to VRF charts. In the case of the Jeppesen IFR charts the numbers refer to the grid minimum offroute altitude (MORA) and they are set at 1,000 feet above the highest point in the grid in normal terrain and 2,000 feet above in mountainous terrain.

On Jeppesen charts the numbers shown are grid MORA altitudes. In those which contain terrain which is 6,000 feet or lower have an obstacle clearance of 1,000 feet. If the MORA altitudes are 7,000 feet or greater, the obstacle clearance is 2,000 feet.

If the Portuguese chart is constructed like the CAA chart (I only used Jepp in Portugal) then the grid numbers are showing the maximum elevation of terrain/known obstacles within the grid square, so the appropriate VFR/IFR safety margin needs to be added, whereas on the Jepp charts this is done already.

The Jepp charts for VFR seemed to be a little lacking in detail for me.

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